1. Field of Invention
The present invention relates in general to the data processing field. More particularly, the present invention relates to providing thermal compensation when measuring surface topographies at elevated temperatures using a non-contact vibration transducer, such as a laser Doppler vibrometer (LDV).
2. Background Art
Unlike traditional contact vibration transducers, laser-based vibration transducers, or laser vibrometers, require no physical contact with a test object (also referred to as a “target”). Typical examples of applications where a laser-based vibration transducer is the preferred choice include remote, mass-loading-free vibration measurements on targets that are difficult or impossible to access. Laser vibrometers are often used for hard disk drive dynamic testing, as well as measuring the surface topographies of hard disk platters, semiconductor wafers, microelectronic chips, and other high-precision test objects.
The laser source used in a laser vibrometer is typically either a HeNe gas laser emitting visible red light or a solid state laser diode emitting infrared light. In both cases, the emitted energy level is low, ensuring a safe instrument that, depending on the type, can be operated with no or minimal safety precautions.
The ability to incorporate advanced, miniaturized optical mirror systems together with the laser source provides the capability to make automated scanning measurements, where a high number of measurement points can be measured consecutively. Non-contact vibration measurements with very high spatial resolution are possible with such a scanning system and can lead to significant improvements in the accuracy and precision of experimental model models. For example, natural frequencies and mode shapes may be extracted from these non-contact vibration measurements, and modal parameters (e.g., modal mass, stiffness and damping) calculated.
The measurement principle used by laser vibrometers is based upon the Doppler effect. When monochromatic laser light is scattered back from a vibrating target, it undergoes a frequency shift proportional to the velocity of the target. This is known as the Doppler effect. As the target moves towards the light source, the back-scattered light undergoes an increase in frequency. On the other hand, as the target moves away from the light source, the back-scattered light undergoes a lowering of the frequency. If the target is vibrating, the frequency of the back-scattered beam will be frequency modulated at the so-called Doppler frequency. The Doppler frequency is directly proportional to the velocity of the target. Therefore, tracking this Doppler frequency provides direct measurement of the velocity of the target relative to the light source. The Doppler effect can be utilized in systems measuring translational (linear) vibration as well as systems measuring torsional (angular) vibration.
A laser Doppler vibrometer (LDV) utilizes the Doppler effect to measure real time microstructural response and surface topography of a test object. This measurement is usually performed at room temperature (RT). However, it may be desirable or necessary to perform this measurement at an elevated temperature. For example, it may be desirable to perform a topographic measurement of a microelectronic chip at an elevated temperature during its manufacture or at its operating temperature. Increasing the temperature at which such topographic measurements are performed causes the back-scattered light in the laser vibrometer to diffract, resulting in erroneous response of the baseline pulse.
It is known that a normalization operation may be utilized to make measurements provided by a laser vibrometer less sensitive to environmental factors. For example, U.S. Pat. No. 5,917,191, entitled “APPARATUS FOR MEASURING SURFACE TOPOGRAPHY”, issued Jun. 29, 1999 to David Cheng, discloses a laser vibrometer that uses the output of a summing amplifier to normalize the output of a differential amplifier. More particularly, the output of the differential amplifier is divided by the output of the summing amplifier. Each amplifier is electrically connected to two abutting photoconductors of a beam detector. This normalization operation is performed to make measurements provided by the photodetectors insensitive to beam intensity, to changes in the reflectivity of the test object, to signal drift, and to environmental factors. However, normalization schemes such as the normalization operation disclosed in the Cheng patent do not compensate for thermal diffraction of the back-scattered light at elevated temperatures in the laser vibrometer.
Therefore, a need exists for a mechanism for providing thermal compensation when measuring surface topography at an elevated temperature using a non-contact vibration transducer, such as a laser Doppler vibrometer.